Making my building blocks modular
Modular assets need quite a lot of planing. First you need to specify a grid size. This might be a floor tile, it also might be bound to a certain measurement like the width of a door frame. Measured from the borders of your base grid, you then need to inset the interior walls and outset the exterior walls. The gap between will be the thickness of the wall. 

The next point you need to consider carefully, is the center of your wall. If your grid starts at either the inside or the outside, your modules won't have the right size. You want to have Blender's grid lines exactly in the middle between your inner and your outer wall (see the green lines in the screenshot below. Also notice the slightly thicker lines of Blender's major Unit grid). 



My walls are exactly two Blender Units wide. The floor tiles will then be perfect squares and also have two Blender units in width and height. The floor will overlap with the wall but it won't show from the outside, since it goes only to half the size of the wall's width. 



There you go, perfectly matching tiles in all directions. 

Here's a .blend file with a very basic setup.



PS: You also want to remove all hidden faces. They are unnecessary in nearly every setup, as long as you don't want to create gaps in the wall, where the sides are visible. The floor tiles in my file are just planes but the principle is the same as it is for the walls.